DUMP TERMINATOR
When routing light on photonic integrated circuit (PIC) chips optical back-reflection and scattering can be highly detrimental to the desired application. Unused ports of optical devices, such as MMI, DC, Y-junction, PD, etc. are a cause for back-reflection and scattering, whereby the scattered light could get picked up by adjacent components, e.g. photodetectors. Management of stray light on the PIC is needed to prevent the undesired coupling between various components and to reduce noise. A dump taper may be used to guide and scatter stray light away from sensitive components or fully absorb the light while maintaining very low reflection from the taper. A doped dump taper may be used to passively absorb light reaching the unused port, thereby eliminating unwanted reflection and scattering. Alternatively, an undoped taper may be used to scatter light away from sensitive components while maintaining very low back-reflection.
This application is a continuation of and claims priority to U.S. patent application Ser. No. 16/255,971, filed Jan. 24, 2019, now allowed, which is hereby incorporated by reference herein in its entirety.
TECHNICAL FIELDThe present invention relates to a dump taper, and in particular to a dump taper for terminating an unused arm of a multi-port optical device to reduce back reflection.
BACKGROUNDWith any high index contrast between waveguide and cladding, such as in silicon (Si) photonic systems or group III-V systems, any open port of a multi-port device, such as multi-mode interference (MMI) coupler, directional-coupler (DC), and Y-junction coupler, will cause strong back-reflection and/or scattering into the cladding. Back reflected or scattered light could get picked up by nearby components, adding to optical noise and cross-talk. In a standard design for a ridge waveguide defined in silicon (Si) and surrounded by a dielectric cladding, e.g. silicon-dioxide (SiO2), the refractive index of Si is ˜3.4 while the refractive index of SiO2 is ˜1.44. The high contrast between the Si and the SiO2 results in reflection of ˜20% (or −7 dB) of the light, if a waveguiding element is abruptly terminated, and the rest will be radiated into the substrate.
An object of the present invention is to overcome the shortcomings of the prior art by providing a dump terminator for controlling scattering of light and/or for absorption of light, while maintaining low reflection (<−30 dB).
SUMMARY OF THE INVENTIONAccordingly, the present invention relates to an optical coupling device comprising:
at least one input port for inputting light;
a coupling region connected to the at least one input port for dividing the light into first and second portions;
a first output port connected to the coupling region for outputting the first portion of light;
a second output port connected to the coupling region for outputting the second portion of light; and
a dump terminator coupled to the end of the second output port for preventing the second portion of light from re-entering the coupling region.
The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:
While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art.
An embodiment of the present invention, illustrated in
The coupling device 101 may be mounted on a substrate 26, and may include top and bottom cladding layers 27 and 28, respectively, as well as cladding on each side of the waveguides, e.g. coupling region 5, first and second input arms 11 and 12, and first and second output arms 21 and 22.
The preferred embodiment includes a Si ridge waveguide with 220 nm height and 500 nm width in silicon-oxide (SiO2) cladding; however, other types of waveguide geometry may be employed, including, but not limited to, rectangular ridge waveguides of varying thickness, multimode bus waveguide, strip-loaded waveguide, W-waveguides and rib waveguides. Different materials, such as Silicon Nitride (SiN), doped glass, Silicon Oxinitride (SiON), III-V materials, such as Gallium-Arsenide (GaAs) and others, may be used. Depending on the application different geometries may be used for different material systems and wavelength range. Typically, the device may be optimized for the telecom C-band (1530-1565 nm); however, minor adjustments to design may be made to work in the O-band (1260-1360 nm) or L-band (1565-1625 nm). The invention applies to other wavelength ranges as well with proper selection of material systems and waveguide geometry.
For simple passive applications, e.g. systems which may not have doping materials or strongly absorbing materials, a version of the terminator may be designed providing low reflection which scatters light away from nearby components to be eventually absorbed by the substrate 26. With reference to
In an alternative embodiment, illustrated in
The direction of the dump terminator 10 relative to the propagation directions of the other ports, e.g. ports 2 and 3, controls the direction of light scattering from the port 4. For example, in
With reference to
An example of a rib dump taper 25b, as in
The same ridge or rib taper 25a and 25b may also be used in the reverse direction to collect and focus a propagating mode in layers above and below the cladding layers 27 and 28, e.g. the substrate 26, into a waveguide mode in the ridge or rib waveguide 22.
The ridge and rib dump tapers 25a and 25b may be designed and optimized using FDTD simulations to maximize transmission into the cladding 27 and 28 and minimize back-reflection into the optical system. In a silicon photonics chip the waveguide material may be comprised of silicon (Si) and the cladding surrounding the waveguide may be silicon-dioxide (SiO2) which has a much lower refractive index, e.g. n=1.44. The adiabatic tapering waveguide section 25a may be based on a ridge waveguide, the cross-section of the which is illustrated in
With reference to
With reference to
For doped waveguide sections 25c and 25d in silicon systems the change in refractive index An and absorption coefficient Δα at 1550 nm is given by:
Δn=−5.4×10−22ΔN1.013−1.53×10−18ΔP0.858
Δα=8.88×10−21ΔN1.167+5.84×10−20ΔP1.109
where ΔN and ΔP are the carrier densities of electrons and holes respectively [cm−3], and Δn and Δα are the changes in refractive index and absorption of Si respectively. For Si n-type dopants such as Phosphorus, Arsenic, Antimony, Bismuth, and Lithium may be used. Choices of p-type dopants include Boron, Aluminium, Gallium and Indium. Doping enables strong absorption in the doped region 25c, 25d, 35c and 35d while keeping the refractive index difference small enough so that reflection at the interface between doped and undoped regions is negligible (<−80 dB). In one particular example: phosphorus doping with carrier concentration of 1×1018 cm−3 is used to achieve absorption of ˜dB/μm, i.e. a 40 μm doped waveguide section 24c or 25d may achieve ˜40 dB of absorption, i.e. absorbs 0.9999 of incident light. Accordingly, the dump terminator 10c or 10d comprises at least 1×1018 cm−3 dopant concentration, and may be at least 30 μm long, preferably at least 40 μm long, and ideally between 30 μm to 50 μm long.
With reference to
With reference to
With reference to
With reference to
The dump terminators 10a to 10i disclosed herein may also be used for any application or optical component, which requires low reflection or directed scattering into a cladding or substrate. For example, when the reflection of an on-chip component is measured, such as MMI, Y-junction, PD, etc., any addition to the ports of the component would add to the device reflection. Even a straight waveguide has a certain amount of back-reflection due to side-wall scattering. The use of absorbing dump terminators, e.g. 10c, 10d, and 10e, may fully absorb unwanted light with minimal back-reflection which minimizes the reflection effect of unused ports on the measurement. Accordingly, any optical device, i.e. with one or more input ports 1 and/or 2 and one or more output ports 3 and/or 4, may be substituted for the coupling region 5 in
Certain applications may require collecting several unused ports from a plurality of devices. One option is to terminate each port with an absorbing dump terminator 10c, 10d, 10e, 10h or 10i, or a scattering dump terminator 10a, 10b, 10f or 10g, which scatters light away from nearby components. However, some applications may prohibit doping near certain devices or too tightly packed to safely scatter light near the unused port. For these specialized applications the best course of action is to route unused light from any unused port to a section of the PIC (photonic integrated circuit) where the light may be either absorbed or safely scattered away into the substrate by dump terminators 10a-10i.
With reference to
The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
Claims
1. A dump terminator device configured to be coupled to a waveguide for minimizing back reflection of light into an optical system, comprising:
- a tapering waveguide section, tapering from an inner wider end proximate the waveguide to an outer thinner outer free end, whereby an effective index of the tapering waveguide section gradually decreases and adiabatically transforms the light from a guided waveguide mode into a leaky mode in cladding surrounding the tapering waveguide section.
2. The device according to claim 1, wherein the tapering waveguide section comprises a rib waveguide.
3. The device according to claim 2, wherein the tapering waveguide section includes a doped waveguide section comprising either positive or negative dopants for increasing an absorption coefficient of the doped waveguide section while minimizing an index difference between the doped waveguide section and an undoped region.
4. The device according to claim 3, further comprising a doped cladding section surrounding the doped waveguide section.
5. The device according to claim 4, wherein the tapering waveguide section includes a subwavelength grating section surrounded by the cladding, in which a fill ratio of waveguide material to cladding material is gradually decreased from an inner end closest to the waveguide to an outer free end for transmitting a propagating mode into the cladding.
6. The device according to claim 3, wherein the tapering waveguide section includes a subwavelength grating section surrounded by the cladding, in which a fill ratio of waveguide material to cladding material is gradually decreased from an inner end closest to the waveguide to an outer free end for transmitting a propagating mode into the cladding.
7. The device according to claim 2, wherein the dump terminator comprises an optically absorbing enclosure on either side of the tapering waveguide section.
8. The device according to claim 7, wherein the optically absorbing enclosure comprises germanium.
9. The device according to claim 1, wherein the tapered waveguide section comprises a doped waveguide section comprising either positive or negative dopants for increasing an absorption coefficient of the doped waveguide section while minimizing an index difference between the doped waveguide section and an undoped region.
10. The device according to claim 9, wherein the doped waveguide section of the tapered waveguide section includes 1×1018 cm−3 dopant concentration.
11. The device according to claim 9, further comprising a doped cladding section surrounding the doped waveguide section.
12. The device according to claim 11, wherein the tapered waveguide section includes a subwavelength grating section surrounded by the doped cladding section, in which a fill ratio of waveguide material to cladding material is gradually decreased from an inner end closest to the waveguide to an outer free end for transmitting a propagating mode into the doped cladding section.
13. The device according to claim 10, wherein the tapering waveguide section includes a subwavelength grating section surrounded by the cladding, in which a fill ratio of waveguide material to cladding material is gradually decreased from an inner end closest to the waveguide to an outer free end for transmitting a propagating mode into the cladding.
14. The device according to claim 1, further comprising a doped cladding section surrounding the tapering waveguide section.
15. The device according to claim 14, wherein the tapered waveguide section includes a subwavelength grating section surrounded by the doped cladding section, in which a fill ratio of waveguide material to cladding material is gradually decreased from an inner end closest to the waveguide to an outer free end for transmitting a propagating mode into the doped cladding section.
16. The device according to claim 1, wherein the tapered waveguide section comprises a subwavelength grating section surrounded by the cladding, in which a fill ratio of waveguide material to cladding material is gradually decreased from an inner end closest to the waveguide to an outer free end for transmitting a propagating mode into the cladding.
17. The device according to claim 6, further comprising an optically absorbing enclosure on either side of the tapering waveguide section.
18. The device according to claim 17, wherein the optically absorbing enclosure comprises germanium.
19. The device according to claim 1, wherein the dump terminator further comprises a bend section redirecting a propagation direction away from the optical system.
20. The device according to claim 1, wherein the tapering waveguide section is 15 μm to 150 μm long.
Type: Application
Filed: Nov 27, 2019
Publication Date: Jul 30, 2020
Patent Grant number: 10989874
Inventors: Yang Liu (Elmhurst, NY), Ruizhi Shi (New York, NY), Tal Galfsky (Morristown, NJ)
Application Number: 16/698,156